Many products that are sensitive to their environment require a barrier that is highly impermeable to water, oxygen and other gases while remaining lightweight and durable. For example, optoelectronic devices require transparent barrier materials to extend their useful operating lives. Currently, glass is used as a transparent barrier material. Unfortunately, glass is often undesirable because it is either too fragile or too heavy or both. Plastics are more lightweight and less fragile materials. Unfortunately, commercially available plastics lack the desired level of environmental resistance for many optoelectronic applications.
For example, to build durable devices, the active elements of polymer-based LEDs may require incorporation of barrier layers with oxygen permeability levels as low as 10−5 cc/m2/day and water vapor permeability levels as low as 10−6 g/m2/day. A 7 mil thick coating of polyethylene teraphthalate (PET) has an oxygen transmission rate of 8.7 cc/m2/day and a water vapor permeability of 10 g/m2/day. State of the art plastics such as Alcar can protect components with oxygen and water vapor permeability levels of about 7 cc/m2/day and 0.016 g/m2/day respectively.
Single barrier coatings of thin films of inorganic materials such as Al, SiO2 Al2O3 and Si3N4 can be vacuum deposited on polymer substrates to improve barrier impermeability. Such single layer coatings can reduce oxygen and water vapor permeability to levels of about 10−3 cc/m2/day and 10−3 g/m2/day respectively.
Multilayer barrier coatings have been developed using a “sandwich” strategy with an inorganic layer is situated between two polymer layers to further improve the aggregate barrier properties. Sheats and coworkers (U.S. Pat. No. 6,146,225) used a 35 nm thick silicon nitride as an inorganic layer and a one micron thick layer of an acrylate as the polymer material to achieve a barrier with a water-vapor permeation rate of 1.8×10−7 g/m2/day, which is about 40 times better than the requirement for most optoelectronic devices. However, this material is not optically transparent, limiting its use to certain applications only.
More recently, Graff and colleagues (U.S. Pat. No. 6,413,645; U.S. Pat. No. 6,573,652 and U.S. Pat. No. 6,623,861) have developed barrier materials using a multi-stack approach where each stack includes a sputter-deposited, 40 nm barrier layer of a metal oxide, metal nitride, or metal carbide, followed by a flash-evaporated, one micron layer of an acrylate polymer or multilayer thin films comprised of flash-evaporated plastic. While these multi-stack barrier films have useful environmental resistance relative to many previously developed materials, their vacuum-based mode of production is time-consuming and relatively expensive, especially for multiple-stack coatings. Further, the vacuum-based deposition methods limit both the area upon which a coating can be placed (the area must be smaller than the deposition chamber), which in turn limits their use for larger area devices. In addition, flash evaporation and sputter deposition do not tend to provide for uniform conformal coatings of large area surfaces, especially for non-planar substrates with inherent curvature (or even three-dimensional barrier-protection targets). It would be desirable to have a multi-layer, transparent, and durable film that provides for uniform, inexpensive and conformal coating of larger areas with effective environmental resistance in a range of environments.
Thus, there is a need in the art, for a barrier film that overcomes the above disadvantages and a corresponding method for making such a film.